Multiple access communication systems are well understood in the art. Multiple access communication systems are designed to provide access to limited communication resources by a plurality of communication units for the purpose of transmitting communication messages, referred to here as packets. The access methodology, referred to as a multiple access protocol, is chosen such that some appropriate set of performance constraints are met. Typical performance constraints include efficiency of communication resource use, communication message delay, and other similar factors. Multiple access protocols can be regarded as belonging to one of two general types, contention and non-contention.
Non-contention protocols are designed such that a communication unit has exclusive use of a communication resource. Time-division multiple access (TDMA) constitutes one such example where the communication resource is divided into a plurality of time frames and then further subdivided into a plurality of time slots, and each communication unit is assigned exclusive use of one or more time slots in each time frame. This type of protocol is inefficient for communication units with substantially infrequent messages since the assigned time slot remains substantially unused in between messages. The practical number of communication units that can be accommodated by such a protocol is also limited by the delay incurred while waiting for the assigned slot, which delay increases proportionally to the total number of communication units having assigned slots.
Contention protocols, such as slotted ALOHA, are characterized by communication units that actively compete with each other to gain access to the communication resource. In slotted ALOHA, a communication resource is divided into a plurality of time slots. A communication unit desiring to send a packet will transmit in a first subsequent time slot, taking care not to transmit outside of the boundaries of that time slot, and then monitor for a collision. If no other communication unit also transmitted in that time slot, the packet transmission is considered successful. (Other factors, such as communication channel noise, may ultimately result in failure of the message, but these other factors are not related to the access protocol.) If one or more other communication units transmitted a packet in the same time slot, generally all transmissions would fail due to collision. Thus contention protocols generally work well for lightly loaded systems, but performance suffers as load increases because the likelihood of collisions also increases. Further, communication messages longer than the time slot duration must be sent in separate time slots and are subject to collision in each time slot used.
Reservation protocols, a sub-class of contention protocols, are known. Reservation protocols attempt to combine certain aspects of contention and non-contention protocols to provide improved performance for a wider variety of communication system conditions. A typical reservation protocol divides a communication resource into a series of fixed-size time frames. These frames are then further divided into a series of time slots. The time slots are comprised of two types, a reservation time slot and a data time slot, with equal numbers of each in each time frame. The reservation time slots are generally smaller than the data time slots and are grouped together at the beginning of each time frame. A communication unit desiring access to the communication resource transmits randomly in one of the reservation time slots. If it successfully avoids contention and is therefore the only unit to transmit in a given reservation time slot, it obtains exclusive access to the associated data time slot occuring later in the time frame.
In one particular reservation protocol (Reservation-ALOHA (R-ALOHA)) when the communication resource is unused, the protocol operates similarly to slotted ALOHA. When a communication unit desires to send a packet, it transmits in one of the unused time slots, referred to as a random access slot. If the transmission is successful, i.e. it does not collide with another transmission, the communication unit is permitted exclusive use of the same time slot in subsequent time frames, referred to as reserved access slots, until the packet is completely transmitted. Thus the initial ALOHA transmission results in a subsequent reservation of a communication resource. Some method of feedback to the communication units regarding the success or failure of initial ALOHA transmissions is necessary in order for this protocol to be effective.
R-ALOHA efficiently accommodates a wide variety of packet frequencies and sizes. Some limitations can be noted, however. The ultimate efficiency of the protocol is governed by the size of the random access portion of a packet relative to the complete packet since only this portion is subject to contention failure. In R-ALOHA, this size is equivalent to a time slot. There are many competing factors that contribute to the determination of time slot duration in the design of a communication system. The result may not contribute to optimum performance. For example, longer time slots increase transmission efficiency because requiste overhead requirements are reduced, but a longer time slot decreases the effectiveness of R-ALOHA. Also, due to the contention for unused time slots in R-ALOHA, several unused slots may need to pass before a successful random access is accomplished. These unused slots represent wasted communication capacity.
Accordingly, a need exists for a multiple access method that provides increased utilization of a communication resource by a plurality of communication units having widely varying communication requirements.